1. Field of the Invention
This invention relates to vent systems used in the storage and transportation of liquids. More particularly, this invention relates to vent systems appropriate to transportation and storage of liquid sulfur.
2. Prior Art
In recent years, due to increased output of sulfur from refining sour crudes into gasoline, efficient ways for transporting and storing enormous quantities of sulfur have had to be developed. Previously, sulfur was transported as a solid in discreet chunks or dispersed in a liquid medium. However, since sulfur can be melted to a liquid at around 250.degree. F., in more recent years it has become more and more common to transport sulfur in a liquid state.
Sulfur in liquid form is usually shipped in tank trucks, railway tank cars, tank barges and tank ships. These tanks are commonly made of steel, which does not corrode if the sulfur is kept hot enough to prevent accumulation of free moisture. To maintain sulfur in a liquid state, the temperature of the sulfur must be maintained high enough so that it melts but not too high as to trigger the formation of high melting allotropic forms. Sulfur is usually transported in tanks having no baffles or other vane like members which would prevent the liquid from slushing about in the tank during transportation, because sulfur on cooling readily plates out.
Containers of liquid sulfur are usually emptied through the bottom and loaded as a liquid through the top. Unfortunately, because of splashing and sublimation, vents to permit introduction of air during the process of unloading often become blocked or plugged. As can be readily imagined, as liquid sulfur is removed from the bottom of a container, air from outside the container must replace the product or liquid sulfur which is removed. Failure to replace the amount of sulfur that is removed from the container with air results in a partial vacuum within the container making additional removal of sulfur more difficult and even dangerous to the structural stability of the container, which for example can be a tank car.
Chipping away at sulfur encrusted vents to ensure the influx of air is potentially dangerous due to the presence of hydrogen sulfide gas, an almost inevitable by-product present in a tank containing liquid sulfur. Other unpleasant and potentially quite dangerous gases present involve sulfur oxides. Sulfur has a freezing point of 114.degree. C. (238.degree. F.) and an approximate melting point of 119.degree. C. (246.degree. F.) with a boiling point of 444.degree. C. (832.degree. F.) Exposure to more than a few hundred parts per million of hydrogen sulfide can cause coughing, eye irritation, loss of sense of smell after 2 to 15 minutes, altered respiration, pain in the eye and drowsiness after 15 to 30 minutes. At 500-700 parts per million, there is risk of loss of consciousness and the possibility of death in one-half to one hour. Excess of 1,000 parts per million can lead to instaneous unconsciousness and death.
If air vents are to be opened by a person seeking to unload a tank car of liquid sulfur, inhalation equipment providing an external source of oxygen must be worn in order to avoid serious health and safety risks.
Efforts to come up with a vent system which is both environmentally safe and does not permit any significant exposure to dangerous vapors such as H.sub.2 S and sulfur oxides, have proved generally unreliable.